Membranes have been used to remove carbon dioxide and other acid/polar gases from natural gas. Membrane processes have been used to serve as a bulk cut separation of carbon dioxide. In such processes the carbon dioxide permeates through the membrane thereby giving rise to a carbon dioxide enriched permeate gas which is vented at low pressure and a hydrocarbon (HC) enriched product gas at high pressure. In addition, there are a variety of carbon dioxide removal processes utilizing two membranes stages. U.S. Pat. No. 4,130,403 provides a method for removing hydrogen sulfide and carbon dioxide from a natural gas by using acid-gas selective membranes. This two-stage process is taught for reducing HC losses. The rich carbon dioxide stream produced may be used in flooding processes for enhanced oil recovery. This staged permeate process is illustrated in FIG. 1.
Additional patents have disclosed the cryo-separation of carbon dioxide from hydrocarbons. In these cases, the membrane is used to increase the efficiency of the main cryogenic separation process. For this processing, all the feed gas has to be cooled to the cryo temperature. Two examples of these schemes are provided in French Patent Application 2917982 and U.S. Pat. No. 4,936,887. French Patent Application 2917982 provides for pretreating natural gas (I) having hydrocarbon, hydrogen sulfide and water that comprises (a) cooling (I) and introducing the cooled natural gas to a separation device (B1) to separate a liquid aqueous phase from (I); (b) contacting (I) with a liquid (11) rich in hydrogen sulfide to obtain a gas (6) that is free of water and an effluent liquid (4); (c) separating the gas, obtained in step (b), to obtain a permeate (8) rich in hydrogen sulfide and a retentate (7) free of hydrogen sulfide; and (d) partially condensing the permeate by cooling to obtain a liquid (10) rich in hydrogen sulfide, and a gas (12) that is free of hydrogen sulfide. Pretreating natural gas (I) having hydrocarbon, hydrogen sulfide and water, comprises (a) cooling (I) and introducing the cooled natural gas to a separation device (B1) to separate a liquid aqueous phase from (I); (b) contacting (I) with a liquid (11) rich in hydrogen sulfide, obtained in step (d), to obtain a gas (6) that is free of water and an effluent liquid (4) rich in water and hydrogen sulfide; (c) separating the gas, obtained in step (b), through a membrane (M) to obtain a permeate (8) rich in hydrogen sulfide and a retentate (7) free of hydrogen sulfide; and (d) partially condensing the permeate by cooling to obtain a liquid (10) rich in hydrogen sulfide, which is recycled to step (b), and a gas (12) that is free of hydrogen sulfide.
U.S. Pat. No. 4,936,887 provides a membrane separation incorporated into a distillation cycle for efficient recovery of carbon dioxide from a stream containing natural gas along with carbon dioxide. Methane and carbon dioxide are separated from a feed stream in a first distillation to produce a process stream containing essentially methane and carbon dioxide and which is substantially free to ethane and higher molecular weight hydrocarbons. The process stream consisting essentially of methane and carbon dioxide is subjected to further distillation to produce a carbon dioxide-rich product stream and a process stream enriched in methane. The methane-enriched process stream is then passed to a membrane separation unit for separating methane and carbon dioxide and for producing a high purity methane product stream.
U.S. Pat. No. 5,647,227 teaches a membrane separation process combined with a cryogenic separation process for treating a gas stream containing methane, nitrogen and at least one other component. The membrane separation process works by preferentially permeating methane and the other component and rejecting nitrogen. The process is particularly useful in removing components such as water, carbon dioxide or C3+ hydrocarbons that might otherwise freeze and plug the cryogenic equipment. In this scheme, the residue stream from the membrane is sent on to the cryogenic separator.
When the carbon dioxide permeate from the first membrane stage contains high hydrocarbon concentrations, these losses can be reduced by re-pressurizing the carbon dioxide permeate and feeding this repressurized carbon dioxide permeate to a second membrane stage. The carbon dioxide permeate from the second membrane stage is vented while the recovered hydrocarbon residue stream is often recycled to be added back to the original hydrocarbon feed methane containing feed gas. These two membrane stage processes have become increasingly important with regarding to the increasing value of the hydrocarbons. However, while the hydrocarbon losses are decreased by using such processes, there is an increase in the separation process costs. Furthermore, the carbon dioxide that is vented in such processes is typically obtained at low pressures so that additional high costs are incurred when the desire is to sequester the carbon dioxide or use it in enhanced oil recovery applications.
U.S. Pat. No. 4,639,257 discloses recovery of carbon dioxide from a gas mixture using a combination of membrane separation and distillation. The process teaches two embodiments. With regard to the preferred embodiment, the membrane separation preferably employs at least two stages with intermediate recompression. The final CO2 concentrated permeate is subjected to distillation to produce liquid CO2 with the required cooling provided by a separate vapor compression refrigeration unit. The process scheme is not integrated in terms of energy requirements making this uneconomical. In the second embodiment, membrane separation is only utilized on the overhead stream from the distillation column.
Accordingly, there is a need to provide a process which not only minimizes the loss of hydrocarbons but also allows for the efficient recovery of the carbon dioxide at high pressure.